This study examines the prevalence of anti-heparin/PF4 antibodies as well as heparin-induced thrombocytopenia in pediatric patients undergoing cardiopulmonary bypass surgery. Specific Aims: - To determine the prevalence of heparin-dependent platelet-reactive antibodies in a pediatric population undergoing cardiopulmonary bypass surgery for congenital heart disease. - To determine the incidence of heparin-induced thrombocytopenia in a pediatric population after exposure to heparin during cardiopulmonary bypass surgery - To examine the role of heparin-induced thrombocytopenia in clinical thrombotic or thromboembolic events occurring in pediatric patients after heparin exposure for cardiovascular procedures Heparin is used frequently as an antithrombotic agent during cardiovascular procedures in patients with congenital heart disease. An increasingly recognized complication of heparin therapy is the development of heparin induced thrombocytopenia (HIT) and related thrombosis. Although the occurence of heparin associated thrombocytopenia has been noted anecdotally in the pediatric population, no study has examined the prevalence of heparin associated antibodies or the incidence of the syndrome in children. Generally thought to be an almost unavoidable consequence of prolonged hospitalization and central venous/arterial monitoring, some portion of postoperative thrombus formation in pediatric patients may represent the HIT syndrome. Heightened recognition of the disorder may be of major clinical significance to avoid complications in patients with congenital heart disease who face repeated exposures to intravenous and high dose heparin during catheterizations and surgical procedures. It will be of import to identify subpopulations at risk for HIT in order to offer alternative anticoagulation strategies. Additional insights into immune mechanisms and pathophysiology of associated vascular thrombosis may derive from investigation of this syndrome in patients of varying age groups, including those without prior heparin exposure and without the presence of superimposed atherosclerotic or other vascular diseases.